The present invention relates to a sulphur-containing ferritic stainless steel that can be used for ferromagnetic parts.
Ferritic stainless steels are characterized by a defined composition, the ferritic structure being in particular ensured, after the composition has been rolled and cooled, by an annealing heat treatment giving them the said structure.
Among the broad families of ferritic stainless steels, defined in particular according to their chromium and carbon contents we mention:                ferritic stainless steels that may contain up to 0.17% carbon. These steels, after the cooling that follows their smelting, have an austenoferritic two-phase structure. However, they may be transformed to ferritic stainless steels after annealing, despite a high carbon content;        ferritic stainless steels whose chromium content is around 11 or 12%. They are quite similar to martensitic steels containing 12% chromium, but differ in their carbon content which is relatively low.        
When steel is hot-rolled, the steel may have a two-phase—ferritic and austenitic—structure. If the cooling is vigorous, for example, the final structure is ferritic and martensitic. If the cooling is slower, the austenite partially decomposes into ferrite and carbides, but with a carbon content richer than the surrounding matrix, the austenite,, when hot, having dissolved more carbon than the ferrite. In both cases, a tempering or annealing operation must therefore be carried out on the hot-rolled and cooled steels in order to generate a completely ferritic structure. The tempering may be performed at a temperature of about 820° C., below the alpha→gamma temperature Ac1, thereby precipitating carbides.
In the field of ferritic steels intended for applications utilizing the magnetic properties, the ferritic structure is obtained by limiting the amount of carbides, and it is for this reason that the ferritic stainless steels developed in this field have a carbon content of less than 0.03%.
Steels that can be utilized for their magnetic properties are known, such as for example those in the document U.S. Pat. No. 5,769,974 which discloses a process for manufacturing a corrosion-resistant ferritic steel and able to reduce the value of the coercive field of the said steel. The compositional ranges presented are very broad and do not define a range for optimizing the properties needed for applications for ferromagnetic parts. The steel used in the process is a steel of the resulphurized type. However, the steel obtained by the process, which contains sulphur, is sensitive to corrosion.
Also known is the patent U.S. Pat. No. 5,091,024 which discloses corrosion-resistant magnetic articles formed from an alloy essentially consisting of a composition having a low carbon content and a low silicon content, that is to say less than 0.03% and 0.5% respectively. However, in the magnetic field, it is important that the steel contains a high silicon content in order to increase the resistivity of the material and reduce eddy currents.
Also known is the French patent application No. 94/06590 (now French Patent No. 2,720,410, corresponding to U.S. Pat. No. 5,496,515), which relates to a ferritic steel with improved machinability for applications in the machining field, but the compositional ranges presented are very broad and do not define a range for optimizing the properties necessary for ferromagnetic parts.